In general, data is written to floppy disks in a series of sectors, angular blocks of the disk, and in tracks, concentric rings at a constant radius, e.g. the HD format of 3½-inch floppy disks uses 512 bytes per sector, 18 sectors per track, 80 tracks per side and two sides, for a total of 1,474,560 bytes per disk. (Some disk controllers can vary these parameters at the user's request, increasing the amount of storage on the disk, although these formats may not be able to be read on machines with other controllers; e.g. Microsoft applications were often distributed on Distribution Media Format (DMF) disks, a hack that allowed 1.68 MB (1680 kB) to be stored on a 3½-inch floppy by formatting it with 21 sectors instead of 18, while these disks were still properly recognized by a standard controller.) On the IBM PC and also on the MSX, Atari ST, Amstrad CPC, and most other microcomputer platforms, disks are written using a Constant Angular Velocity (CAV)—Constant Sector Capacity format.[citation needed] This means that the disk spins at a constant speed, and the sectors on the disk all hold the same amount of information on each track regardless of radial location.
However, this is not the most efficient way to use the disk surface, even with available drive electronics.[citation needed] Because the sectors have a constant angular size, the 512 bytes in each sector are packed into a smaller length near the disk's center than nearer the disk's edge. A better technique would be to increase the number of sectors/track toward the outer edge of the disk, from 18 to 30 for instance, thereby keeping constant the amount of physical disk space used for storing each 512 byte sector (see zone bit recording). Apple implemented this solution in the early Macintosh computers by spinning the disk slower when the head was at the edge while keeping the data rate the same, allowing them to store 400 kB per side, amounting to an extra 160 kB on a double-sided disk.[citation needed] This higher capacity came with a serious disadvantage, however: the format required a special drive mechanism and control circuitry not used by other manufacturers, meaning that Mac disks could not be read on any other computers. Apple eventually gave up on the format and used constant angular velocity with HD floppy disks on their later machines; these drives were still unique to Apple as they still supported the older variable-speed format.
The Commodore 64/128
Commodore started its tradition of special disk formats with the 5¼-inch disk drives accompanying its PET/CBM, VIC-20 and Commodore 64 home computers, the same as the 1540 and 1541 drives used with the later two machines. The standard Commodore Group Code Recording scheme used in 1541 and compatibles employed four different data rates depending upon track position (see zone bit recording). Tracks 1 to 17 had 21 sectors, 18 to 24 had 19, 25 to 30 had 18, and 31 to 35 had 17, for a disk capacity of 170 kB (170.75 KB). Unique among personal computer architectures, the operating system on the computer itself was unaware of the details of the disk and filesystem; disk operations were handled by Commodore DOS instead, which was implemented as firmware on the disk drive.
Eventually Commodore gave in to disk format standardization, and made its last 5¼-inch drives, the 1570 and 1571, compatible with Modified Frequency Modulation (MFM), to enable the Commodore 128 to work with CP/M disks from several vendors. Equipped with one of these drives, the C128 was able to access both C64 and CP/M disks, as it needed to, as well as MS-DOS disks (using third-party software), which was a crucial feature for some office work.
Commodore also offered its 8-bit machines a 3½-inch 800 kB disk format with its 1581 disk drive, which used only MFM.
The GEOS operating system used a disk format that was largely identical to the Commodore DOS format with a few minor extensions; while generally compatible with standard Commodore disks, certain disk maintenance operations could corrupt the filesystem without proper supervision from the GEOS kernel.
The Atari 8-bit line
The combination of DOS and hardware (810, 1050 and XF551 disk drives) for Atari 8-bit floppy usage allowed sectors numbered from 1 to 720. The DOS' 2.0 disk bitmap provides information on sector allocation, counts from 0 to 719. As a result, sector 720 could not be written to by the DOS. Some companies used a copy protection scheme where "hidden" data was put in sector 720 that could not be copied through the DOS copy option. Another more-common early copy-protected scheme simply did not record important sectors as "used" in the FAT table, so the DOS Utility Package (DUP) did not duplicate them. All of these early techniques were thwarted by the first program that simply duplicated all 720 sectors.
Later DOS versions (3.0 and later 2.5) and DOS systems by third parties (i.e. OSS) accepted(and formatted) disks with up to 960 and 1020 sectors, resulting in 127KB storage capacity per disk side on drives equipped with double-density heads (i.e. not the Atari 810) vs. previous 90KB. That unusual 127K format allowed sectors 1-720 to still be read on a single-density 810 disk drive, and was introduced by Atari with the 1050 drive with the introduction of DOS 3.0 in 1983.
A true 180K double-density Atari floppy format used 128 byte sectors for sectors 1-3, then 256 byte sectors for 4-720. The first three sectors contain code that signals the drive to switch into double-density mode. While this 180K format was developed by Atari for their DOS 2.0D and their (canceled) Atari 815 Floppy Drive, that double-density DOS was never widely released and the format was generally used by third-party DOS products. Under the Atari DOS scheme, sector 360 was the FAT sector map, and sectors 361-367 contained the file listing. The Atari-brand DOS versions and compatible used three bytes per sector for housekeeping and to link-list to the next sector.
Third-party DOS systems added features such as double-sided drives, subdirectories, and drive types such as 1.2Mb and 8". Well-known 3rd party Atari DOS products included SmartDOS (distributed with the Rana disk drive), TopDos, MyDos and SpartaDOS.
The Commodore Amiga
The pictured chip, codenamed Paula, controlled floppy access on all revisions of the Commodore Amiga as one of its many functions.
The Commodore Amiga computers used an 880 kB format (eleven 512-byte sectors per track) on a 3½-inch floppy. Because the entire track was written at once, inter-sector gaps could be eliminated, saving space. The Amiga floppy controller was much more flexible than the one on the PC: it did not impose arbitrary format restrictions, and foreign formats such as the IBM PC could also be handled (by use of CrossDos, which was included in later versions of Workbench). With the correct filesystem software, an Amiga could theoretically read any arbitrary format on the 3.5-inch floppy, including those recorded at a differential rotation rate. On the PC, however, there is no way to read an Amiga disk without special hardware or a second floppy drive,[28][29] which is also a crucial reason for an emulator being technically unable to access real Amiga disks inserted in a standard PC floppy disk drive.
Commodore never upgraded the Amiga chip set to support high-density floppies, but sold a custom drive (made by Chinon) that spun at half speed (150 RPM) when a high-density floppy was inserted, enabling the existing floppy controller to be used. This drive was introduced with the launch of the Amiga 3000, although the later Amiga 1200 was only fitted with the standard DD drive. The Amiga HD disks could handle 1760 kB, but using special software programs it could hold even more data. A company named Kolff Computer Supplies also made an external HD floppy drive (KCS Dual HD Drive) available which could handle HD format diskettes on all Amiga computer systems .
Because of storage reasons, the use of emulators and preserving data, many disks were packed into disk-images. Currently popular formats are .ADF (Amiga Disk File), .DMS (DiskMasher) and .IPF (Interchangeable Preservation Format) files. The DiskMasher format is copyright-protected and has problems storing particular sequences of bits due to bugs in the compression algorithm, but was widely used in the pirate and demo scenes. ADF has been around for almost as long as the Amiga itself though it was not initially called by that name. Only with the advent of the Internet and Amiga emulators has it become a popular way of distributing disk images. IPF files were created to allow preservation of commercial games which have copy protection, which is something that ADF and DMS unfortunately cannot do.
The Electron, BBC Micro and Acorn Archimedes
The British company Acorn used non-standard disk formats in their 8-bit BBC Micro and Acorn Electron, and their successor the 32-bit Acorn Archimedes. Acorn however used standard MFM disk controllers. The original disk implementation for the BBC Micro stored 100 KB (40 track) or 200 KB (80 track) per side on 5¼-inch discs in a custom format using the Disc Filing System (DFS).
For their Electron floppy disk add-on added, Acorn picked 3½-inch disks and developed the Advanced Disc Filing System (ADFS). It used double-density recording and added the ability to treat both sides of the disc as a single drive. This offered three formats: S (small) — 160 KB, 40-track single-sided; M (medium) — 320 KB, 80-track single-sided; and L (large) — 640 KB, 80-track double-sided. ADFS provided hierarchical directory structure, rather than the flat model of DFS. ADFS also stored some metadata about each file, notably a load address, an execution address, owner and public privileges and a "lock" bit. Even on the eight-bit machines, load addresses were stored in 32-bit format, since those machines supported 16 and 32-bit co-processors.
The ADFS format was later adopted into the BBC line upon release of the BBC Master. The BBC Master Compact marked the move to 3½-inch disks, using the same ADFS formats.
The Acorn Archimedes added D format, which increased the number of objects per directory from 44 to 77, and increased the storage space to 800 KB. The extra space was obtained by using 1024 byte sectors instead of the usual 512 bytes, thus reducing the space needed for inter-sector gaps. As a further enhancement, successive tracks were offset by a sector, giving time for the head to advance to the next track without missing the first sector, thus increasing bulk throughput. The Archimedes used special values in the ADFS load/execute address metadata to store a 12-bit filetype field and a 40-bit timestamp.
RISC OS 2 introduced E format, which retained the same physical layout as D format, but supported file fragmentation and auto-compaction. Post-1991 machines including the A5000 and Risc PC added support for high-density discs with F format, storing 1600 KB. However, the PC combo IO chips used were unable to format discs with sector skew, losing some performance. ADFS and the PC controllers also support extended-density disks as G format, storing 3200 KB, but ED drives were never fitted to production machines.
With RISC OS 3, the Archimedes could also read and write disk formats from other machines, for example the Atari ST and the IBM PC. With third party software it could even read the BBC Micro's original single density 5¼-inch DFS disks. The Amiga's disks could not be read as they used unusual sector gap markers.
The Acorn filesystem design was interesting because all ADFS-based storage devices connected to a module called FileCore which provided almost all the features required to implement an ADFS-compatible filesystem. Because of this modular design, it was easy in RISC OS 3 to add support for so-called image filing systems. These were used to implement completely transparent support for IBM PC format floppy disks, including the slightly different Atari ST format. Computer Concepts released a package that implemented an image filing system to allow access to high density Macintosh format disks.
4-inch floppy diskettes
In the mid-80s, IBM developed a 4-inch floppy diskette, the Demidiskette. This program was driven by aggressive cost goals, but missed the pulse of the industry. The prospective users, both inside and outside IBM, preferred standardization to what by release time were small cost reductions, and were unwilling to retool packaging, interface chips and applications for a proprietary design. The product never appeared in the light of day, and IBM wrote off several hundred million dollars of development and manufacturing facility.
Auto-loaders
IBM developed, and several companies copied, an autoloader mechanism that could load a stack of floppies one at a time into a drive unit. These were very bulky systems, and suffered from media hangups and chew-ups more than standard drives,[citation needed] but they were a partial answer to replication and large removable storage needs. The smaller 5¼- and 3½-inch floppy made this a much easier technology to perfect.
Floppy mass storage
A number of companies, including IBM and Burroughs, experimented with using large numbers of unenclosed disks to create massive amounts of storage. The Burroughs system used a stack of 256 12-inch disks, spinning at high speed. The disk to be accessed was selected by using air jets to part the stack, and then a pair of heads flew over the surface as in any standard hard disk drive. This approach in some ways anticipated the Bernoulli disk technology implemented in the Iomega Bernoulli Box, but head crashes or air failures were spectacularly messy. The program did not reach production.
2-inch floppy disks
2-inch Video Floppy Disk from Canon.
A small floppy disk was also used in the late 1980s to store video information for still video cameras such as the Sony Mavica (not to be confused with current Digital Mavica models) and the Ion and Xapshot cameras from Canon. It was officially referred to as a Video Floppy (or VF for short).
VF was not a digital data format; each track on the disk stored one video field in the analog interlaced composite video format in either the North American NTSC or European PAL standard. This yielded a capacity of 25 images per disk in frame mode and 50 in field mode.
The same media were used digitally formatted - 720 kB double-sided, double-density - in the Zenith Minisport laptop computer circa 1989. Although the media exhibited nearly identical performance to the 3½-inch disks of the time, they were not successful. This was due in part to the scarcity of other devices using this drive making it impractical for software transfer, and high media cost which was much more than 3½-inch and 5¼-inch disks of the time.
Ultimate capacity and speed
Floppy disk drive and floppy media manufacturers specify an unformatted capacity, which is, for example, 2.0 MB for a standard 3½-inch HD floppy. It is implied that this data capacity should not be exceeded since exceeding such limitations will most likely degrade the design margins of the floppy system and could result in performance problems such as inability to interchange or even loss of data.
User available data capacity is a function of the particular disk format used which in turn is determined by the FDD controller manufacturer and the settings applied to its controller. The differences between formats can result in user data capacities ranging from 720 KB (.737 MB) or less up to 1760 KB (1.80 MB) or even more on a "standard" 3½-inch HD floppy. The highest capacity techniques require much tighter matching of drive head geometry between drives; this is not always possible and cannot be relied upon. The LS-240 drive supports a (rarely used) 32 MB capacity on standard 3½-inch HD floppies[citation needed]—it is, however, a write-once technique, and cannot be used in a read/write/read mode. All the data must be read off, changed as needed and rewritten to the disk. The format also requires an LS-240 drive to read.
Some special hardware/software tools, such as the CatWeasel floppy disk controller and software, which claim up to 2.23 MB of formatted capacity on a HD floppy. Such formats are not standard, hard to read in other drives and possibly even later with the same drive, and are probably not very reliable. It is probably true that floppy disks can surely hold an extra 10–20% formatted capacity versus their "nominal" values, but at the expense of reliability or hardware complexity.
DSED 3½" FDDs introduced by Toshiba in 1987 and adopted by IBM on the PS/2 in 1994[12] operate at twice the data rate and have twice the capacity of DSHD 3½" FDDs[31]. The only serious attempt to speed up a 3.5” floppy drive beyond 2X was a 10X floppy drive. X10 accelerated floppy drive. It used a combo of RAM and 4X spindle speed to read a floppy in less than 6 seconds vs. the over 1 min time it normally takes.
3½-inch HD floppy drives typically have a transfer rate of 1000 kilobits/second (minus overhead such as error correction and file handling). (For comparison a 1X CD transfers at 1200 kilobits/second (maximum), and a 1X DVD transfers at approximately 11,000 kilobits/second.) While the floppy's data rate cannot be easily changed, overall performance can be improved by optimizing drive access times, shortening some BIOS introduced delays (especially on the IBM PC and compatible platforms), and by changing the sector:shift parameter of a disk, which is, roughly, the numbers of sectors that are skipped by the drive's head when moving to the next track.
This happens because sectors are not typically written exactly in a sequential manner but are scattered around the disk, which introduces yet another delay. Older machines and controllers may take advantage of these delays to cope with the data flow from the disk without having to actually stop.
Usability
One of the chief usability problems of the floppy disk is its vulnerability. Even inside a closed plastic housing, the disk medium is still highly sensitive to dust, condensation and temperature extremes. As with any magnetic storage, it is also vulnerable to magnetic fields. Blank floppies have usually been distributed with an extensive set of warnings, cautioning the user not to expose it to conditions which can endanger it.
Users damaging floppy disks (or their contents) were once a staple of "stupid user" folklore among computer technicians. These stories poked fun at users who stapled floppies to papers, made faxes or photocopies of them when asked to "copy a disk", or stored floppies by holding them with a magnet to a file cabinet. Also, these same users were, conversely, often the victims of technicians' hoaxes. Stories of them being carried on Subway/Underground systems wrapped in tin-foil to protect them from the magnetic fields of the electric power supply were common (for an explanation of why this is plausible, see Faraday cage). The flexible 5¼-inch disk could also (folklorically) be abused by rolling it into a typewriter to type a label, or by removing the disk medium from the plastic enclosure used to store it safely.
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